Tissue implanted glucose needle electrodes: early sensor stabilisation and achievement of tissue-blood correlation during the run in period

In this study, two approaches were utilised to reduce surface fouling and enhance operational reliability of tissue implanted glucose sensors. The first, Open Microflow, used an open cannula incorporated needle electrode. Here, the sensor surface was subjected to slow flow of protein-free fluid (chelate containing isotonic buffer or saline) directly at the implant site. Positive outcomes of this included reduced stabilisation time (≤90 min), accelerated 'pick up' of tissue glucose changes after intravenous injections of glucose or insulin without the usual reported lag, and high correlation between tissue and blood glucose values under dynamic conditions avoiding the need for in vivo calibration and a reduced surface fouling. Open Microflow fluid composition was important for performance. A range of isotonic solutions were tested; NaCl alone, 4 mM KCl and NaCl, 0.74% (w/v) Na2HPO4 with NaCl or insulin (2 U/1 ml) gave partial correlation with blood glucose, while best agreement was seen with EDTA/phosphate buffer. A second approach evaluated an outer membrane barrier comprising polyurethane precursor (Trixene SC762®) and non-ionic surfactant (2% (v/v) Triton X100). This modified polyurethane conferred good haemo- and tissue biocompatibility with exposure to whole blood confirming low fouling, and for tissue no evident drift during acute monitoring.